Processes and materials that are, or are suspected to be, ecologically damaging are increasingly unacceptable and alternative ecologically safer solutions are demanded. Conventionally, polyurethanes are manufactured by the reaction of organic materials containing two or more hydroxyl groups with other organic materials (monomers, dimers, trimers and oligomers) containing two or more isocyanate groups. Such isocyanates are highly toxic and are produced from by an even more toxic starting material, phosgene. Secondly, polyurethane production is prone to an undesirable side-reaction between the isocyanate groups and moisture, which yields carbon dioxide within the polyurethane mass, resulting in bubbles of carbon dioxide being trapped in the finished material, causing the polyurethane to be porous. Conventional polyurethanes are unstable in the presence of water and have a poor chemical resistance to aqueous solutions of acids and alkalis, which limits their use.
A series of relatively recent patents disclose polyurethane or polyurethane-epoxy hybrid compositions based on the reaction between, on the one hand, oligomer(s) bearing cyclocarbonate groups or both epoxy groups and cyclocarbonate groups and, on the other hand, amines. More particularly, U.S. Pat. No. 5,340,889 discloses a method for producing linear non-isocyanate polyurethanes from the reaction of cyclocarbonate derivatives and amines.
SU-1,754,748 deals with an epoxy-based composite material for flooring applications that includes an oligomeric cyclocarbonate modifier with a monofunctional hardener (aminophenol) for the modifier, resulting in an epoxy-based material with immobilized non-isocyanate oligo-urethane moieties.
U.S. Pat. No. 5,175,231 and U.S. Pat. No. 6,495,637 disclose a multi-step process for the preparation of a network comprising non-isocyanate polyurethane links for use as a hardener for epoxy resins.
U.S. Pat. No. 4,785,615 discloses polymer compositions containing urethane groups that are capable of being crosslinked by crosslinking agents, prepared without the use of isocyanates by reacting polyamino compounds with polycarbonates and if appropriate, reacting the product further with polycarboxylic acids to form a series of products intended for use as adhesives and paints, especially aqueous baking paint formulations and aqueous curable paints that can be deposited by anaphoresis.
U.S. Pat. No. 6,120,905 discloses hybrid non-isocyanate network polyurethanes formed by crosslinking at least one cyclocarbonate oligomer with an average functionality of from about 2.0 to about 5.44 and at least one of these cyclocarbonate oligomers consists from about 4 to about 12% w/w of terminal epoxy groups, with one amine oligomer. The patent also relates to methods of making hybrid non-isocyanate polyurethane networks for use in composite materials containing a fibre reinforcement (glass fibre, carbon fibre, basalt fibre and mixtures thereof), or a particulate reinforcement, e.g. a metal oxide or a metal aluminate salt.
EP-1,020,457 and U.S. Pat. No. 6,407,198 relate to the synthesis of polyfunctional polycyclocarbonate oligomers. The polycyclocarbonates are prepared by the reaction of oligocyclocarbonates containing terminal epoxy groups with primary aromatic diamines and they were used for the preparation of hybrid materials for adhesives, sealants, composite materials, coatings or synthetic leather. It is mentioned that pigments and fillers (e.g. barium sulphate, titanium dioxide, silica, aluminate cement and ferrous oxides pigments) can be also added in the preparation of adhesives compositions
EP-1,070,733 relates to the synthesis of polyaminofunctional hydroxyurethane oligomers and hybrids prepared therefrom. It states that it is impossible to form composite polyurethane/epoxy resins by curing a composition containing both epoxy groups and cyclocarbonate groups with a hardener containing primary amine groups because of the competition between the epoxy and cyclocarbonate groups for reaction with the primary amines. It therefore proposes a curable composition containing an oligomer containing both a cyclocarbonate ring and an epoxy ring.
Micheev V. V. et al. report (Lakokrasochnye Materialy I Ikh Primenenie, 1985, 6, 27-30) that co-curing of oligomeric cyclocarbonate resins and epoxies with polyamines yields products with enhanced properties over monolithic non-isocyanate-based polyurethanes but they do not include any comparative example in their study.
In 1990, researchers at TOYOTA Central Research & Development Laboratories (Japan) [a] Fukushima, Y. et all., J. Inclusion Phenom., 1987, 5, 473, b] Fukushima, Y, et all., Clay Miner., 1988, 23, 27, c] Usuki, A. et all., J. Mater. Res., 1993, 8, 1174, d] Yano K. et all., J. Polym. Sci. Part A: Polymer Chem., 1993, 31, 2493, e) Kojima, Y. et all., J. Polym. Sci. Part A: Polymer Chem., 1993, 31, 983] disclosed the enhancement in mechanical properties of nylon-clay nanocomposites.
Researchers have concentrated on four nanoclays as potential nanoscale particles: a) hydrotalcite, b) octasilicate, c) mica fluoride and d) montmorillonite. The first two have limitations both from a physical and a cost standpoint. The last two are used in commercial nanocomposites. Mica fluoride is a synthetic silicate, montmorillonite (MMT) is a natural one. The theoretical formula for montmorillonite is:M+y(Al2-yMgy)(Si4)O10(OH)2*nH2O
Ionic phyllosilicates have a sheet structure. At the Angstrom scale, they form platelets, which are 0.7-1 nm thick and several hundred nanometers (about 100-1000 nm) long and wide. As a result, individual sheets have aspect ratios (Length/Thickness, L/T) varying from 200-1000 or even higher and, after purification, the majority of the platelets have aspect ratios in the 200-400 range. In other words, these sheets usually measure approximately 200×1 nm (L×T). These platelets are stacked into primary particles and these primary particles are stacked together to form aggregates (usually about 10-30 μm in size). The silicate layers form stacks with a gap in between them called the “interlayer” or “gallery”. Isomorphic substitution within the layers (Mg2+ replaces Al3+) generates negative charges that are counterbalanced by alkali or alkaline earth cations situated in the interlayer. Such clays are not necessarily compatible with polymers since, due to their small size, surface interactions such as hydrogen bonding become magnified. Thus, the ability to disperse the clays within some resins is limited and at the beginning, only hydrophilic polymers (e.g. PVA) were compatible with the clays because silicate clays are naturally hydrophilic. But, it was found that the inorganic cations situated in the interlayer can be substituted by other cations. Cationic exchange with large cationic surfactants such as alkyl ammonium-ions, increases the spacing between the layers and reduces the surface energy of the filler. Therefore, these modified clays (organoclays) are more compatible with polymers and form polymer-layered silicate nanocomposites. Various companies (e.g. Southern Clays (of 1212 Church Street, Gonzales, Tex. USA 8629), Sud Chemie, Nanocor, etc.) provide a whole series of both modified and natural nano clays, which are montmorillonites. Apart from montmorillonites, hectorites and saponites are the most commonly used layered silicates.
A nanocomposite is a dispersion, often a near-molecular blend, of resin molecules and nanoscale particles. Nanocomposites can be formed in one of the following three ways: a) melt blending synthesis, b) solvent based synthesis and c) in-situ polymerization, as is known in the art.
There are three structurally different types of nanocomposites: 1) intercalated (individual monomers and polymers are sandwiched between silicate layers) 2) exfoliated (a “sea” of polymer with “rafts” of silicate), and 3) end-tethered (a whole silicate or a single layer of a silicate is attached to the end of a polymer chain).
Glass transition temperature is a fundamentally important property of polymers since it is the temperature at which properties of the polymer change radically. In some instances, it is desirable to have a high glass transition temperature for a polyurethane polymer.
Gel time is also an important production parameter and fast gel times allow a polymer to be manufactured or formed more rapidly. Gel times and cure times are obviously related and both will be referred to in the present specification. In addition, fast cure times allow adhesives to set quickly to produce the desired bond.